ID 1389 NDE OF COMPOSITES USING THE ULTRASONIC METHOD OF SIMULTANEOUS VELOCITY, THICKNESS AND PROFILE SCAN INTRODUCTION

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1 ID 1389 NDE OF COMPOSITES USING THE ULTRASONIC METHOD OF SIMULTANEOUS VELOCITY, THICKNESS AND PROFILE SCAN David K. Hsu and Dong Fei Center for Nondestructive Evaluation, Ioa State University, Ames, Ioa , USA SUMMARY: This paper applies the simultaneous velocity and thickness scan technique to image small changes in velocity and thickness simultaneously and extends this technique to map out the surface elevation contours and cross-sectional profiles of a sample. Special considerations for achieving accurate and reliable velocity, thickness and profile images are discussed in detail. This technique for nondestructive evaluation and materials characterization is demonstrated on to industrially relevant materials: (1) composite laminates containing foreign objects and anomalies and (2) plasma sprayed thermal barrier coatings. KEYWORDS: ultrasonic velocity measurement; thickness gauging and profiling; ultrasonic imaging. INTRODUCTION Ultrasonic velocity has been used extensively in nondestructive evaluation (NDE) to characterize material properties. For a plate of uniform and knon thickness, the usual ultrasonic scan based on the time-of-flight (TOF) of ultrasonic pulses can provide a spatial image ( C-scan ) of the velocity. Hoever, there are numerous cases here the thickness of the sample is non-uniform or cannot be accurately measured, then it is necessary to separate the contribution of the velocity and the thickness of the sample in the TOF image and determine the spatial variation of the velocity and the thickness simultaneously. Such a capability can serve as a useful NDE tool for cases here a component in service has suffered changes in both thickness and material property. It can also be applied to cases here the velocity is only knon approximately and the precise spatial variation of the thickness or the surface profile of the component are the information sought after in the NDE test. The Simultaneous Velocity and Thickness (SVT) method has been developed over the last 15 years or so by a number of researchers for various NDE [1-3] and medical ultrasonic [4] applications. The SVT scan can be done in immersion using a transmission setup or a pulseecho setup ith the aid of a reflector behind the sample. Automated scans of the SVT method have been reported for both plate [5] and cylindrical [6] geometry; the technique can also be implemented using squirters [3] outside an immersion tank. In most cases the velocity as deduced from the TOF data acquired using peak detection, but Fourier transform and the phase of the signal have also been used [2]. Recently the SVT method has been expanded to include dispersion effects and to provide the ultrasonic attenuation [7]. Applications of the SVT scan method to industrial materials and components have been reported as ell [6, 8]. In this paper, the pulse-echo mode of the SVT scan technique as implemented using a SONIX ultrasonic immersion scan system to map out small changes in ultrasonic velocity and sample thickness. This techniq ue as also extended to generate the surface elevation contours and cross-sectional profiles. Special cares ere exercised in order to achieve high accuracy and precision. Such a Simultaneous Velocity, Thickness and Profile (SVTP) imaging technique as first demonstrated using an aluminum test sample ith machined surface contours and constant material properties. The SVTP technique as then applied to detect small changes in velocity, thickness and profiles of to industrially relevant materials: 1

2 Transducer Water t 1 t 2 t m1 Sample t m2 t d F d B d Amplitude m 1 m 2 m 3 sample in path sample not in path Reflector Time Fig.1 The pulse-echo measurement configuration and typical received aveforms for simultaneous determination of ultrasonic velocity, sample thickness and surface contours. (1) carbon epoxy composite laminates containing foreign objects and anomalies and (2) plasma sprayed thermal barrier coatings. Basic Principle MEASUREMENT METHOD In this ork the simultaneous velocity and thickness determination is accomplished in a pulse-echo setup ith the aid of a reflector plate. As shon in Fig. 1, hen the transducer is positioned at a given location over the reflector plate and operates in the pulse-echo mode, to RF aveforms are of interest, as shon in Fig. 1, respectively hen the sample is in the ultrasonic path (solid curve) and hen the sample is not in the path (gray curve). Without the sample in the path, the transducer receives an echo from the reflector, hich is labeled in Fig. 1. When the sample is in the path, the transducer receives multiple echoes due to reverberation in the sample and multiple echoes bounced off the reflector plate, also ith reverberation in the sample. As shon in the solid curve in Fig. 1, the first set of echoes, labeled 1, 2, 3,, are directly from the sample, and the second set, labeled m 1, m 2, m 3,, are from the reflector. It has been shon in reference [3] that by measuring t 1, t 2, t m1, and t, respectively the TOF for echoes 1, 2, m 1 and, the longitudinal ave group velocity, v, and the thickness, d, at the local position of the sample can be determined simultaneously according to the folloing to equations: d t tm1 v = v + 1, (1) t2 t1 [( t t ) + ( t )] = ( v / 2) t, (2) m1 2 1 here v is the sound velocity in ater at the temperature of the experiment, hich can be found from a number of references in the literature [9], or be predetermined. Note that the four times-of-flight are gouped into differences of time; this is advantangeous as any uncertainties in the zero of time due to the unknon triggerring instant and the signal delay in the electronic measurement system ill be subtracted out. 2

3 The TOF of the front surface echo can be used to determine the distance beteen the transducer and the front surface of the sample, d F, as follos, d 1 v t 2 =. (3) F 1 Because the sample thickness d has already been obtained from the SVT measurement (Eq. (2)), the distance beteen the transducer and the sample s back surface, d B, can also be easily deduced using the relationship: d = d d. (4) B F + In the SVTP imaging, the transducer is scanned over the sample surface, all the locally measured thicknesses and longitudinal ave velocities respectively form the velocity image and the thickness image. The locally measured distances beteen the transducer and the front and back surfaces of the sample respectively give the front and back surface contours. Such a SVTP imaging method can be conveniently implemented in a commerical ultrasonic scan systems in three steps. First, e place the sample and make a sample scan (TOF scan ith the sample in the ultrasonic path) to get the TOF images for echoes 1, 2 and m 1. Then e remove the sample and make a reference scan (TOF scan ithout the sample in beteen the transducer and the reflector) to get the TOF image for echo. Finally, e read the output TOF image files from the above to steps, convert the image data back to times of flight for each echo based on the gate setting, and calculate the images of velocity, thickness, and surface contours using Eqs. (1), (2), (3), and (4). The cross-sectional profile images of the sample are further generated from the surface contour images. Experimental Considerations Although the above measurement principle and procedure are quite straightforard, special attention must be given to the folloing items in order to efficiently achieve accurate and reliable images of the velocity, thickness and sample profiles. Gate settings. It is of obvious importance that in a SVTP measurement all the measured TOFs, including t 1, t 2, and t m1 in the sample scan and t in the reference scan, must have a common zero-time reference. A simple ay to ensure this is not to use any folloer gate. Gate length is also an important parameter. A gate should be long enough to cover the possible TOF variation of the selected echo during the scan. A gate, hoever, should not interfere ith the neighboring gates and should not be too long. An excessively long gate degrades the time resolution of the TOF measurement. Peak detector settings. There are a number of TOF timing methods applicable to the SVT measurement in a commercial ultrasonic scan system [6]. In this ork, the peak method, either positive peak, negative peak or absolute peak, has been used for its simplicity. When the positive peak detection is chosen, the TOF of the largest positive peak in the gate is recorded; similarly for the negative setting. The absolute setting records the TOF of the peak hose absolute amplitude is the largest in the gate. One must therefore be careful about the polarity of the peak detection setting to ensure that the TOF of the correct peak is recorded and used in the velocity, thickness and profile calculation. When the sample material has lo attenuation and dispersion and the sample has no dramatically irregular cross-sectional shapes, each echo should have a ell-defined peak of the proper polarity. The absolute peak detection ould be the best choice for such a situation because it ould alays locate the dominant peak of the correct polarity ithin the gate and no special attention is needed for the pulse polarity. Hoever, if the sample material is highly attenuative and dispersive, the echo aveform may have comparable positive and negative peaks due to the pulse distortion. If the absolute peak detection ere used, the peak chosen 3

4 for the TOF measurement may sitch back and forth beteen the positive and negative peaks, depending on the noise and other non-ideal factors. Such sitching can cause considerable noise in the resulting velocity, thickness and profile images. Therefore, in cases here the aveform is noisy but the anticipated peak polarity is knon, it is important that the correct peak polarity be selected and that the absolute peak detection should be avoided. Effects of geometric setup. There are a number of non-ideal conditions in the geometric SVTP scan setup that can cause measurement errors to an unacceptable degree; it is therefore important that e pay attention to the effects of those conditions in order to achieve highprecision images. For example, the locus of the transducer in a scan forms the scan surface. Ideally such a surface should be a plane, but in reality it can have a non-planar profile, due to static factors such as the drooping of the scan bridge, and dynamic factors such as the vibration of the search tube. The scan surface profile directly affects the results of the front and back surface contours of the sample because both of them use the scan surface as a reference. The scan surface profile ill not affect the velocity and thickness results as long as the scan surfaces in the sample scan and the reference scan are the same. If not, the measured thickness image ill be affected and the velocity image ill also be affected accordingly. A flat, stable, and reproducible scan system is important in achieving high precision velocity, thickness and profile images. Another example is the reflector surface. The reflecting surface should also be parallel to the scan plane and remains the same in the sample scan and reference scan. In practice a frequently encountered problem is that the position and angle of the reflector could be slightly disturbed due to the placement and removal of the sample. Such disturbances directly affect the velocity, thickness and profile results. Therefore, in measurements here highly accurate results are required and the sample is heavy, it is important to make sure that the disturbance of the reflector due to sample placement and its loading effect is kept at a very minimum. Other factors include the transducer and sample orientations. The transducer should be normal to the scan surface and the sample should be placed such that it is essentially parallel to the scan surface despite the small variations in both front and back surface contours. Sound velocity in ater. From Eqs. (1), (2), (3), and (4) e can see that the calculated velocity, thickness and surface contours are proportional to the sound velocity in ater. An inaccurate value of the sound velocity in ater ould therefore directly affect the calculated results. For example, the sound velocity in distilled ater is m/s and m/s, respectively at 20.0 C and 24.0 C [9]. The percent difference beteen these to velocities is about 0.8%. The difference caused by a temperature variation of one-tenth of one degree Celsius in this temperature range is about 0.02%. When accurate absolute values of velocity, thickness and surface contours are required, it is important to measure the ater temperature in experiment to one-tenth of a degree Celsius and use the corresponding sound velocity in ater. An alternative ay of choosing echoes. From the term t 2 - t 1 in Eqs. (1) and (2) one can see that only the time difference beteen to consecutive echoes from the sample is required. In principle, such a time difference can be measured using any pair of consecutive echoes in the first and second sets shon in Fig. 1. Particularly, if the purpose of the measurement is only to get the velocity and thickness images, it is advantageous to use echoes m 1 and m 2 instead of echoes 1 and 2 for the folloing reasons. First, e no only need to acquire, store and process three TOFs, t m1, t m2 and t, instead of four TOFs that are required in Eqs. (1) and (2). Second, m 1 and m 2 are of the same polarity, hile echoes 1 and 2 have opposite polarity. Third, the TOFs of echoes m 1 and m 2 are not sensitive to the distance beteen the transducer and the sample, hich makes the sample level adjustment (or the concentric alignment in a circular turn-table scan) is much less critical and is therefore easier to achieve. Finally, the amplitudes of echoes m 1 and m 2 are usually much smaller than those of echoes 1 and 2. It is often difficult to have adequate gain for echo m 1 ithout saturating echoes 1 and 2. By using only echoes m 1 and m 2, this dynamic range problem can be avoided. Hoever, it should be 4

5 also noted that echoes m 1 and m 2 may not be suitable for measurements on samples ith high attenuation and dispersion. Because the aves for echoes 2 and m 1 have propagated through the sample only tice hile echo m 2 four times, echo m 2 has usually suffered more severe pulse distortion than echoes 2 and m 1. Also, hen the sample material is attenuative and dispersive, echo m 2 may not have a dominant peak ith a ell-defined polarity hile echoes 2 and m 1 may still maintain a dominant peak ith a ell-defined polarity. In that case, echoes 1, 2, and m 1 ould still be the better choice for getting lo-noise velocity and thickness images. An alternative to a reflector plate. Instead of using a large reflector plate and making a reference scan, a small planar reflector large enough to intercept the entire ultrasonic beam may be used. This small reflector is positioned at the desired distance from the transducer, orientated perpendicular to the beam, and ganged together ith the transducer via a U- shaped yoke. The depth of the yoke should be sufficient to accommodate the sample size and the sample may be mounted horizontally or vertically. During a SVTP scan, both the transducer and the reflector move together to collect data at different scan positions. There are several advantages in this alternative setup. First, the reflector ill alays remain perpendicular to the beam and the alignment is unaffected by the placement or removal of the sample. Second, because the distance beteen the transducer and the reflector is fixed, t is not a function of scan position. There is therefore no need for a reference scan ; a single TOF measurement for t and a sample scan ill provide all the times of flight needed for constructing the velocity, thickness and surface profile images. This method can be applied equally ell to the scan of flat plates and circular cylinders. An obvious limitation of this setup is that the sample size ill be limited by the depth of the U-shaped yoke. RESULTS AND DISCUSSION In the folloing, the extended profiling ability of the SVT imaging technique as first demonstrated using an aluminum sample ith surface features and constant material properties. The SVTP imaging technique as then applied to to industrial material systems using samples ith spatially varying ultrasonic velocity and/or thickness and profile to demonstrate its value in nondestructive testing and material property characterization. The to materials ere: (1) oven carbon epoxy laminates containing various foreign objects and anomalies and (2) plasma sprayed thick thermal barrier coatings that ere mixtures of metal (NiCrAlY) and ceramics (Zirconia). Validation of the Technique We machined an aluminum test sample ith a dimension of mm 50.9 mm 9.50 mm. The sample had designed recess on both the front and back surfaces. As shon in Fig. 2, the sample s front surface had a 12.7-mm ide, 1.35-mm deep rectangular groove and an 18.9-mm diameter, 0.76-mm deep circular indent. The back surface had a 25.4-mm ide, 1.53-mm deep rectangular groove and a 12.6-mm diameter, 1.02-mm deep circular indent. The sample surfaces ere not polished. The SVTP scan as performed ith a spherically focussed transducer (Panametrics V311, 10 MHz central frequency, 12.7 mm in diameter, and mm in focal length) at a step size of 0.01 inch (2.54 mm) in both x and y directions. The sample as placed in the focal zone. The resulting images for the velocity, thickness, front surface contour and back surface contour are given in Figs. 2, (c), (d) and (e), respectively. A sketch of the top vie of the sample contour as superimposed on all the figures as dotted lines for geometrical comparison. As expected, the velocity image (Fig. 2 ) shos that the sample had a uniform velocity distribution and the value as consistent ith that measured by conventional 5

6 methods. It should be noted that the corners at the grooves and indents disrupted the ultrasonic beam and Back, 1.53 deep Front, 1.35 deep Front, 18.9 diameter, 0.76 deep Back, 12.6 diameter, 1.02 deep Y (mm) (c) Y (mm) (d) (e) Fig. 2 The aluminum test sample and its SVTP imaging results. dimensions of the sample, velocity image in mm/µs, (c) thickness image in mm, (d) front surface contour in mm, and (e) back surface contour in mm. Dotted lines indicate the scaled top vie of the sample. led to edge effects; also the data ere invalid at the to ends over the support rollers. The thickness image given in Fig. 2 (c) shos clearly that the sample had five distinctly different thickness regions. Hoever, based on the thickness image alone, one ould not be able to determine hether such thickness variations ere due to the displacement of the front surface or the back surface, or both. The resulting front surface contour image, as shon in Fig. 2 (d), shos that the sample s front surface had a rectangular groove and a circular indent. The resulting back surface contour image (Fig. 2 (e)) indicates that the back surface had a ider rectangular groove and a smaller circular indent. In both images, the location and size of the surface contour features matched ell it h the sample geometry (indicated by the dotted lines). On the back surface contour image e can faintly see the edges of the rectangular groove and circular indent on the front surface due to the edge effects. 6

7 Application to Composite Laminates Containing Foreign Objects and Anomalies Composite laminates containing different types of artificially embedded foreign objects and anomalies have also been used as samples to test the utility of the SVTP imaging technique in fla detection. The first laminate sample as a 4-mm-thick oven carbon/epoxy laminate. A 38.1-mm diameter circular nylon bag as embedded in the laminate. Figures 3 and are the SVTP velocity and thickness images, respectively; (c) and (d) are respectively the conventional BSE TOF and amplitude images, for comparison. The velocity image in Fig. 3 clearly shoed the presence of the circular foreign object as a region of slightly loer velocity, hich is a result of the stiffness reduction caused by the embedded nylon bag. The thickness image, Fig. 3, shos a diffused region of slightly increased thickness but did not clearly sho a ell-defined circular area. This as because of the draping effect of the plies around the edge of the embedded defect. In contrast, the conventional BSE amplitude and TOF images shoed different information. The TOF image in Fig. 3 (c) only shoed a very faint image of the fla. The amplitude image in Fig. 3 (d), hoever, shoed a distinct ring of much reduced amplitude along the edge of the embedded nylon bag. This as caused by the scattering and beam disruption due to the resin-rich area and ply draping around the edge of the inclusion. In the second carbon/epoxy laminate sample, a 38.1-mm diameter hole as punched in the second ply before lay-up. This circular area as then filled ith a disk of neat resin to simulate a resin-rich area. The velocity image produced by the SVTP imaging technique (Fig. 4 ) shos clearly the reduction of velocity in the defective circular area. The thickness image (Fig. 4 ) again shos a diffused bulge due to the embedded resin. The conventional Y (mm) Y (mm) (c) (d) 7

8 Fig. 3 A comparison of the conventional C-scan images and the SVTP images on a composite laminate ith an embedded nylon bag: velocity image in mm/µs, thickness image in mm, (c) BSE TOF image in µs, and (d) BSE amplitude image in full screen height. Y (mm) (c) (d) Y (mm) Fig. 4 A comparison of the conventional C-scan images and the SVTP images on a composite laminate ith a simulated resin-rich area: velocity image in mm/µs, thickness image in mm, (c) BSE TOF image in µs, and (d) BSE amplitude image in full screen height. BSE amplitude and TOF images, shon respectively in Figs. 4 (c) and (d), give some indication about the edge of the defective area, but did not clearly sho the defective area itself. Application to Thick Thermal Barrier Coating (TTBC) Samples The SVTP imaging technique as also applied to measure the small variations in both velocity and physical dimensions of plasma sprayed TTBC samples ith different eight composition of metal (NiCrAlY) and ceramic (Zirconia). Here e only sho the results on one TTBC sample. This sample had 60% metal and 40% ceramic, and as about 2.3 mm thick and 89 mm in diameter. A 10-MHz spherically focussed transducer (Panametrics V311, 12.7 mm in diameter and mm in focal length) as used to obtain high spatial resolution images. As shon in Fig. 5, the velocity image (Fig. 5 ) shos that the sample had a relatively higher velocity (around 3.8 mm/µs) in the central part and a loer velocity (about 3.4 mm/µs) near the edges. The difference as about 10%. The thickness image (Fig. 5 ) also shos about a 10% thickness variation in the sample. The front and back surface contour images are shon in Figs. 5 (c) and (d), respectively. The highest point of the front surface of the sample as chosen as the reference point for vieing the surface contour results. From the to surface contours e can see that the pattern in the thickness images as mainly due to the variation in the back surface contour. Three cross-sectional profiles at y = 25, 50 and 75 mm are given in Figs. 6, and (c), respectively. The gray level in the three images 8

9 indicates the local longitudinal velocity in the thickness direction. Such plots vividly sho the variation of both the material property and physical dimension in the sample s crosssections. Y (mm) Y (mm) (c) (d) Fig. 5 The SVTP images of the thick thermal barrier coating sample: velocity image in mm/µs, thickness image in mm, (c) front surface contour in mm, and (d) back surface contour in mm. Z (mm) Z (mm) (c) Z (mm) 9

10 Fig. 6 Three cross-sectional profile images of the thick thermal barrier coating sample, respectively for y = 25 mm, y = 50 mm, and (c) y = 75 mm. The grayscale in each image corresponds to the local velocity in mm/µs. CONCLUSIONS This study has extended the simultaneous velocity and thickness imaging technique to produce the sample profiles and has demonstrated that this technique can be easily implemented in a commercial scan system ith one transducer and a reflector plate behind the sample. By paying careful attention to the details of the measurement, this technique can be used to produce accurate and reliable images of not only thickness and velocity, but also sample surface contours and cross-sectional profiles. Several application examples have demonstrated that this technique may be a valuable tool for nondestructive testing as ell as material property characterization of both metals and composites. ACKNOWLEDGEMENTS This ork as supported by the NSF Industry-University Cooperative Research Center for Nondestructive Evaluation at Ioa State University. The authors gratefully acknoledge Greig Happoldt of Caterpillar for providing the thermal barrier coating samples. We also thank Dan J. Barnard for useful discussions regarding imaging the surface profiles. One of the authors, Dong Fei, acknoledges the support of a graduate assistantship from the Institute for Physical Research and Technology at Ioa State University. REFERENCES 1. L. H. Pearson and D. S. Gardiner, in Proc. of 15 th NDE Symposium, eds. D. W. Moore and G. A. Matzkanin (NTIA center, TX, 1985), p V. Dayal, An automated simultaneous measurement of thickness and ave velocity by ultrasound, Expt. Mech., 32(2), , (1992). 3. D. K. Hsu and M. S. Hughes, Simultaneous ultrasonic velocity and sample thickness measurement and application in composites, J. Acoust. Soc. Am. 92(2), , (1992). 4. I. Y. Kuo, B. Hete, and K. K. Shung, A novel method for the measurement of acoustic speed, J. Acoust. Soc. Am. 88, (1990). 5. M. S. Hughes and D. K. Hsu, An automated algorithm for simultaneously producing velocity and thickness images, Ultrasonics 32(1), 31-37, (1994). 6. D. J. Roth and D. A. Farmer, Scaling up the single transducer thickness-independent ultrasonic imaging method for accurate characterization of microstructural gradients in monolithic and composite tubular structures, NASA Report TM P. He, Measurement of acoustic dispersion using both transmitted and reflected pulses, J. Acoust. Soc. Am. 107(2), , (2000). 8. D. K. Hsu, D. Fei, R. E. Shannon and V. Dayal, Simultaneous Velocity and Thickness Imaging by Ultrasonic Scan, to appear in Revie of Progress in Quantitative Nondestructive Evaluation, edited by D. O. Thompson and D. E. Chimenti (AIP, 2001). 9. J. R. Lovett, Comments concerning the determination of absolute sound speeds in distilled and seaater, J. Acoust. Sco. Am. 45, (1969). 10